Cell Growth Dynamics in Embryonic and Adult Mouse Thyroid Revealed by a Novel Approach to Detect Thyroid Gland Subpopulations

Background: The thyroid is composed of endocrine epithelial cells, blood vessels, and mesenchyme. However, no data exist thus far on absolute cell numbers, relative distribution, and proliferation of the different cell populations in the developing and mature thyroid. The aim of this study was therefore to establish a flow cytometry protocol that allows detection and quantification of discrete cell populations in embryonic and adult murine thyroid tissues. Methods: Cell-type anti-mouse specific antibodies were used for erythroid cells (Ter119), hematopoietic cells (CD45), epithelial cells (EpCam/CD326, E-cadherin/CD324), thyroid follicular cells and C-cells (Nkx2-1), endothelial cells (Pecam/CD31, Icam-1/CD54), and fibroblasts (PDGFRa/CD140a). Proliferating cells were detected after labeling with 5-bromo-2-deoxyuridine (BrdU). For flow cytometry analyses, micro-dissected embryonic (E) and adult thyroids were pooled (E13.5, n=25; E15.5, n=15; E17.5, n=15; adult, n=4) in one sample. Results: The absolute parenchymal cell numbers per mouse thyroid (M SD), excluding the large number of CD45(+) and Ter119(+) cells, increased from 7425 +/- 1338 at E13.5 to 271,561 +/- 22,325 in adult tissues. As expected, Nkx2-1(+) cells represented the largest cell population in adult tissues (61.2 +/- 1.1%). Surprisingly, at all three embryonic stages analyzed, thyroid follicular cells and C-cells accounted only for a small percentage of the total thyroid cell mass (between 4.7 +/- 0.4% and 9.4 +/- 1.6%). In contrast, the largest cell population at all three embryonic stages was identified as PDGFRa/CD140a(+) fibroblasts (61.4 +/- 0.4% to 77.3 +/- 1.1%). However, these cells represented the smallest population in adult tissues (5.2 +/- 0.8%). Pecam/CD31(+) endothelial cells increased from E13.5 to E15.5 from 3.7 +/- 0.8% to 8.5 +/- 3.0%, then remained stable at E17.5 and adult tissues. Proliferation rates were sizable during the entire organogenesis but differed between cell populations, with distinct proliferative peaks at E13.5 in epithelial cells (32.7 +/- 0.6% BrdU(+) cells), and at E15.5 in endothelial cells (22.4 +/- 2.4% BrdU(+) cells). Fibroblasts showed a constant proliferation rate in embryonic tissues. In adult tissues, BrdU(+) cells were between 0.1% and 0.4% in all cell types. Conclusions: Using a novel flow cytometry-based method, a previously unobserved highly dynamic growth pattern of thyroid cell populations during embryogenesis was uncovered. This approach will provide a useful new tool for cell function analyses in murine thyroid disease models.